igb: add support for set_rx_mode netdevice operation

This patch adds support for the set_rx_mode netdevice operation so that igb
can better support multiple unicast addresses.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>